Since only a portion of the oil contained in a subterranean oil-bearing formation can be recovered by primary methods, it has been conventional practice to employ various secondary and tertiary recovery techniques to produce additional quantities of oil. One of the most widely practiced secondary techniques is the displacement of oil from the formation with a driving fluid such as flood water ejected for that purpose. Normally, in carrying out a water flooding procedure, a series of input wells spaced apart from one or more production wells are drilled into and opened to the oil-producing formation. Flood water, which often contains salt, is injected into the reservoir under pressure, forcing some of the oil towards the production well or wells where the oil is recovered.
Tertiary techniques have been developed which seek to reduce the oil-water interfacial tension, either by injecting aqueous surfactant solutions into the oil formation, or by forming surfactants in situ. In this regard, crude petroleum is known to contain varying amounts of surfactant-yielding materials. These have generally been thought of as being simply saponifiable materials such as petroleum acids which can react with alkaline materials to form soaps that reduce the interfacial tension between the crude petroleum and water. Accordingly, the use of alkaline water-flooding, to activate such surfactants as a tertiary recovery mechanism, has been extensively investigated.
Various investigators, such as Reisberg, in U.S. Pat. No. 3,111,984, and Gilchrist et al, in U.S. Pat. No. 3,344,858, have proposed adding soap-forming organic acids, generally containing about 6-40 carbon atoms per molecule, to the oil formation, either prior to or subsequent to injection of alkaline solution, with the aim of forming additional surfactant to aid in lowering the surface tension of the formation oil. However, it is now apparent that the alkaline waterflooding method for enhanced oil recovery is a complex process. Cooke, Williams, and Kolodzie, J. Petrl. Tech. 26 (12), 1365 (1974) found that though in situ oxidation with air further increases the acid number of a given crude oil, this artificially created high-acid-number crude oil could not successfully be flooded with alkaline water. Also, investigators have identified compounds other than carboxylic acids, such as phenols or porphyrins, as beneficial to oil recovery due to the low interfacial tensions they exhibit.
It has also been found that the ability of an alkaline flooding procedure to lower interfacial tension is transient, and is often lost during the time of oil-alkali contact, with the result that substantial amounts of oil, otherwise recoverable by alkaline floodwater procedures remain behind in the oil formation. Furthermore, even when a formation is subjected to repeated injections of floodwater to the point of depletion of recoverable oil, there still remains a substantial amount of oil trapped within the formation; in many cases, substantially more oil remains within the formation than is recovered by alkaline flooding.
The present invention provides a process for recovering oil from a formation in addition to that which is recoverable by alkaline flooding. The process is useful prior to alkaline flooding to enhance the effectiveness of an alkaline flooding procedure, as well as to enable recovery from a formation which has been substantially depleted by alkaline flooding. In accordance herewith, at least part of the oil in the oil-bearing formation is potentiated for further or increased recovery by alkaline flooding by introducing into the formation an aqueous solution of acid selected from mineral acids and organic acids having less than 5 carbon atoms per molecule. Thereafter, alkaline floodwater is introduced to displace potentiated oil from the formation to one or more production wells.
It will be appreciated that the acid referred to is not the type of acid which has been used by prior investigators for the formation of surfactants in situ; they are not soap-forming acids. Also the acids are not being used merely to disintegrate rocks to facilitate the ease of flow; such a process is commonly referred to as acidization. Although the mechanism is not fully understood, the acid appears to reinduce lost activity in the oil formation, with the result that additional alkaline flooding is effective in displacing additional amounts of oil. Also, oil-bearing formations which have not been exposed to alkaline can be made more active by initial contact with the acid, thereby making the first alkaline flooding procedure more effective.
In greater detail, a tertiary process is provided for the recovery of oil from a substantially watered-out subterranean oil-bearing formation. The formation is penetrated by spaced injection and production wells, in accordance with usual procedure, and an aqueous solution of an acid as aforesaid is introduced into the formation. Preferably, at least one pore volume of the acid solution is passed through the formation at a concentration of at least 0.05 normal. The potentiated oil in the formation can then be water-flooded to substantially decrease acidity, following which alkaline flood water is introduced resulting in displacement of oil to the production well or wells. The cycle can be repeated for the recovery of still additional oil. In each case, to conserve acid or alkaline, prior acid or alkaline, prior to each step of acidification and alkaline-flooding, the oil-bearing formation is washed with flood water, e.g. ground water or saline water such as brine.